Humans drive coloration shifts in Bahamian mosquiotofish

Back in August we published a paper in Evolutionary Applications that we’ve been working on for a few years. It’s finally out, here’s the link: Link to .pdf. But read below for a brief summary:

Humans alter the environment in many ways. The consequences of these ecological changes are usually obvious and familiar: species abundances and the nature of their interactions change. These types of consequences are the focus of much of the conservation ecology research that we and others do. But in a recently published study we took a slightly different tack, we ask whether the deterioration of our natural environment could cause evolutionary changes in wild populations. More specifically:

Do species that persist following habitat alteration adapt through evolution to their novel, altered environments?

Figure 1. The ecological effects of blocking a tidal creek are very consistent. While there’s usually a large number of changes (e.g., salinity, pH, water color) the most obvious is that predators are lost from the blocked portion (right side of the road) and prey fishes like mosquitofish flourish.

This study was motivated by a growing recognition that evolution can happen rapidly(*). But whether wild organisms are adapting to the massive changes humans are affecting is still largely unknown, though widely suspected. So in this study, we decided to address this outstanding question with a large-scale study of a common type of environmental alteration in The Bahamas, (and elsewhere) aquatic fragmentation, and the organisms that are affected. Below, I break this study down by focusing on it’s findings in two parts. Part I discusses the main question we sought to answer – is there change? But in Part II, I discuss a secondary question – is this change predictable across species?

Figure 2. Bahamian mosquitofish (Gambusia spp.) vary a lot in color. Above are four male mosquitofish from Abaco Island, The Bahamas that typify variation among tidal creeks. The brightly-colored orangish red dorsal fin is immediately noticeable, but the number of small black dots on the fins and body also vary considerably among populations. These fins are often used by males during courtship displays, presumably to attract the attentions of females. The role of the black spots is less well understood.

Part I. Human-driven evolution in the wild.

In The Bahamas, tidal creek blockage disconnects these ecosystems from the ocean. This causes local populations of predator fishes: barracuda, snapper and needlefish, to go extinct (Figure 1). Therefore, to test our overall question, we focused our attention on a small, numerous fish that tolerates (and in some cases, flourishes) in the novel ecosystems created by fragmentation, Bahamian mosquitofish (Figure 2). With the loss of these predators we thought that these tough, rapidly-reproduction fish might evolve.

Methods

To test our hypothesis we collected fish from 47 mosquitofish populations located throughout The Bahamas (Figure 3). About half of these populations were from tidal creeks that were blocked (aka, fragmented), usually by roads. We then measured the coloration (the number of spots on fish fins, and the color of their dorsal fin (the top one)) for 5-10 adult males from each population. We focused on coloration because we thought that these traits would be likely candidates for evolution since predators exert strong evolutionary pressures on them (predators tend to attack brightly-colored males).

Figure 3. Our study included populations of fish from six islands and covered a large portion of The Bahamas Archipelago.

Findings!

Our main finding was that mosquitofish from blocked creeks were different from fish in unblocked/natural tidal creeks! Throughout the archipelago (Long Island to Grand Bahama) mosquitofish were different. Fish from blocked creeks had less spotting on their fins (Figure 4). These results followed my impression of fish that I see in the field all the time, fish from blocked creeks look quite a bit different. COOL!

Figure 4. The number of dorsal-fin spots was lower for populations in blocked creeks suggesting that they had evolved to have fewer spots in fragmented creeks than their neighbors in the natural creeks.

PART II. How predictable is evolution?

Because the study encompassed such a large area – six islands in the archipelago – we actually had three different species of mosquitofish in our sample. These three species, all endemic to The Bahamas, are almost certainly sister species, meaning that they are all very closely related and have only recently speciated (1-5 million years ago). This allowed us to ask a different question – does each species respond similarly to the effects of blocking? Or, does each species exhibit a unique response?

So, when we include this evolutionary information (i.e., species identity) in our study it turns out that while all mosquitofish species tend to show decreases in the number of fin spots, the fin coloration (how red the fins are) of each species actually responds very differently to tidal creek blockage (Figure 5).

Figure 5. The effect of fragmenting tidal creeks differed among the three sister species. Species differed in whether there was a shift, but they also differed in the direction of change: G. manni was lighter in blocked creeks, while G. sp was redder in blocked creeks.

Importance

You might ask, ‘What does this all mean?’ What I think it means is that wild populations are increasingly exposed to human-altered habitats via climate change, development, harvesting for meat, and a remarkable diversity of other ways. These changes don’t just affect the types and numbers of species persisting; rather, these pervasive ecological shifts cause populations to evolve as well. However, predicting the direction of evolution is bound to be difficult.

If, for example, conservation management of species or ecosystems involves incorporating evolutionary processes, then we will probably have to consider a broader range of information than perhaps we currently expect.

* What I mean by rapidly is that wild populations of organisms can evolve on timescales that we can actually observe and measure. To some extent, this is not a huge surprise. Some well-known and important examples of rapid evolution include the emergence of antibiotic resistant bacteria and pesticide resistant insects

Share This Post!

I am an evolutionary ecologist in the Department of Ecology and Evolutionary Biology at the University of Connecticut. My research investigates how basic ecological interactions control fundamental biological processes such as sexual selection, communication, and predation.